Information on EXT port

Does anyone have additional details on the DS4 EXT port? It appears to be the same as Sony’s headphone and remote jack on their minidisc players (referenced as J301, part no. 1-793-288-62), but I can’t seem to find any additional information going down that route, either. Is it a custom Sony part? I imagine it is. Is the male plus or female receptacle orderable anywhere?